Light olefins serve as feed materials for the production of numerous chemicals. Light olefins have traditionally been produced through the processes of steam or catalytic cracking of hydrocarbons such as derived from petroleum sources. Fluidized catalytic cracking (FCC) of heavy hydrocarbon streams is commonly carried out by contacting relatively high boiling hydrocarbons with a catalyst composed of finely divided or particulate solid material. The catalyst is transported in a fluid-like manner by transmitting a gas or vapor through the catalyst at sufficient velocity to produce a desired regime of fluid transport. Contact of the heavy hydrocarbons with the fluidized catalyst results in the cracking reaction.
FCC processing is more fully described in U.S. Pat. Nos. 5,360,533, 5,584,985, 5,858,206 and 6,843,906. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
The FCC reactor serves to crack gas oil or heavier feeds into a broad range of products. Cracked vapors from an FCC unit enter a separation zone, typically in the form of a main column, that provides a gas stream, a gasoline cut, light cycle oil (LCO), heavy cycle oil (HCO), and clarified oil (CO) components. The gas stream may include hydrogen and C1 and C2 hydrocarbons, and liquefied petroleum gas (“LPG”), i.e., C3 and C4 hydrocarbons.
There is an increasing need for light olefins such as propylene for the production of polypropylene, propyl benzene, cumene and the like. Research efforts have led to the development of an FCC process that produces or results in greater relative yields of light olefins, such as propylene. Such processing is more fully described in U.S. Pat. No. 6,538,169.
A conventional FCC process produces a combined propylene/propane stream. The recovery and purification of propylene from the combined propylene/propane is accomplished via a sequence of distillation operations. The sequence consists of distillation columns to separate both lower and higher boiling components from propylene and generally includes a distillation operation to separate a mixed stream of propane and propylene into a propylene product or “polymer grade” propylene, which can be used for polymer manufacturing in a downstream operation. The propane/propylene separation by distillation is both energy and capital intensive due to the relative volatility of species to be separated, feed composition, and product purity requirements of “polymer grade” propylene.
Because of the energy consumption requirements of splitter columns in general, splitter column configurations for similar boiling point materials that reduce utility consumption are desirable given increasing energy costs and a general need to reduce CO2 emissions associated with fossil fuel consumption.